You can also test as you go. For example, install the antenna and test it, then install the coax and test it, then, if you have adapters, lightning arresters, or anything else to add, install those and test again. The purpose of doing this would be to ensure nothing you add to your antenna system causes any problems along the way.
Your most important reading, though, will be the final one, where the antenna system connects to the radio. If you had any flaky readings along the way, hopefully you would have made any necessary corrections, so the final reading should be good (or the last thing you added could be causing a problem).
Overall, though, you want to check the antenna system as a whole. The test as you go suggestion is mostly to help rule out problems as you're building the antenna system.